A constant preoccupation in the automotive sector is that of fuel consumption and the ecological impact of the vehicle in particular due to its emissions of greenhouse-effect gases such as CO2 or due to toxic gases such as NOx, for example. In order to reduce fuel consumption, automobile manufacturers have been attempting, on the one hand, to make the propulsion engines more efficient and, on the other hand, to reduce the consumption of the equipment of the vehicle.
An important factor in the consumption of a vehicle is determined by the wind loading or the aerodynamics of the vehicle.
Specifically, the aerodynamics of a motor vehicle is an important characteristic since it particularly influences the fuel consumption (and therefore pollution) and also the performance, in particular acceleration performance, of said vehicle.
In particular, drag or aerodynamic resistance to forward travel plays a decisive role, in particular at higher speeds, since drag varies as a function of the square of the speed of movement of the vehicle.
According to the models used in fluid mechanics, it is possible for example to quantify the drag force which is exerted on a motor vehicle with the aid of a reference area S. In a first approximation, the drag force, termed Fx, is equal to q*S*Cx, where q denotes the dynamic pressure (q=½ρ*V2, ρ denoting the density of the air and V the speed of the vehicle with respect to the air), Cx denoting a coefficient of drag specific to the vehicle.
The reference area used for a motor vehicle usually corresponds to its frontal area. It will therefore be understood that, in order to reduce drag, it is necessary to aim at reducing the reference area.
A more detailed analysis of the aerodynamic phenomena has also made it possible to bring to light the decisive role of the vehicle wheels.
Specifically, the wheels may considerably increase the aerodynamic resistance since they generate turbulence when the air flow strikes the rotating wheel. At high speeds, it has been demonstrated that the front wheels can contribute up to a value of 30% to the reference area.
Specifically, when a motor vehicle is moving, the air in which it travels is deflected according to the profile of the vehicle. The air thus deflected particularly reaches the wheel housing. The wheel housing is a cavity formed in the body of the vehicle and surrounding a wheel (this corresponds to the fender of the vehicle). The wheel housing performs a number of functions. It limits in particular (by retaining them) the projections of water, of mud or of other materials on which the wheel is likely to circulate and which it can be led to expel during its rotation. The air reaching the wheel housing circulates in particular in the narrow space separating the wheel from the wheel housing. It is known that, as this takes place, turbulence forms around the wheel revolutions and creates an aerodynamic brake.
It is known to place a fixed deflector in front of a motor vehicle wheel. Such a fixed deflector, which can take the form of a skirt (often of about 5 cm in height), makes it possible to reduce the turbulence in the wheel housing.
However, such a fixed deflector risks being damaged when crossing obstacles (sidewalk, speed-reducing device of the speed hump type, etc.).